Friction stir spot welding (FSSW) is a solid-state welding technique that creates a spot weld without melting the work pieces. In an FSSW process, a rotating tool with a contoured tip is plunged against two or more work pieces, such as flat sheet metal pieces, such that the resulting friction joins the work pieces together. During this process, the tip of the rotating tool penetrates through the top work piece and enters into the lower work piece. As the tip of the rotating tool exits the faying surface of the top work piece and enters the faying surface of the lower work piece, the rotating tool can cause the upper work piece to lift off of and deflect away from the lower work piece. This, in turn, can create an undesirable space or gap between the two work pieces where material can flow into.
With reference to FIG. 1, there is shown a weld assembly 10 having upper and lower work pieces 12, 14 that exhibit the above-explained phenomenon; notice how the faying surface of upper work piece 12 near weld 16 is not flush with the faying surface of lower work piece 14. It can be undesirable to have a deflected arrangement like this, as the deflected work piece can interfere with other components, etc. Contrast this deflected condition with weld assembly 20, which includes flat upper and lower work pieces 22, 24 joined by weld 26. In this example, the faying surfaces of upper and lower work pieces 22, 24 are flat and are in flush contact with one another.